The Arachnida

The Arachnida include the terrestrial
chelicerates
that everyone is
familiar with, and that nearly everyone would rather not be too familiar
with: spiders (Araneae), ticks and mites (Acari), and scorpions
(Scorpiones). Arachnids also include a number
of less familiar taxa: Opiliones (harvestmen or daddy-longlegs);
Thelyphonida (whip-scorpions); Pseudoscorpiones
(false scorpions); and many others. Most
are predators, and some are venomous. All are terrestrial, except for some
mites and spiders that have become secondarily aquatic.